Why use negative feedback!!!

I would like to y use negative feedback in a converters?

bryanwizard said:

I would like to y use negative feedback in a converters?

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hi,
the true objective of negative feedback is insensitiviity

You can find a good explanation in Thomas. Lee's book(rf)

Negative feedback can lower sensetiviy to noise and enlarge bandwidth.

laglead said:

Negative feedback can lower sensetiviy to noise and enlarge bandwidth.

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laglead, how can the negative feedback lower sensitivity to noise? could you explain more?

there is noway for feedback to lower the system's sensitivity to noise.

it will lower the noise if it is inside the system, eg. power supply noise or distortion within. Further analysis is described in the Sergio Franco textbook. I read it there.

changing the in/out impedances, GBW tradeoff, distortion improvement,
don't agree it will improve the noise performance though. In general, it
will add more noise.

The use of negative feedback is to minimize the error in the system...

In full cycle, the system must have a compensation circuit in order to prevent positive feedback in the system...

Futhermore, in the end... Error or noise are minimals...

Negative feedback reduces noise as the noise at the output gets divided by the (1+AB). Also its useless if you operate an amplifier with very less bandwidth because you will not be able to process a large set of frequencies. So you need to enlarge bandwidth ,and this is achieved by using feedback and reduction of gain .Although negative feedback amplifiers cause other problems like instability if not properly designed, by far their advantages outnumber their disadvantages.

amarnath

negative feedback can lower noise sensitivity in an indirect way. With feedback, you can tradeoff other cirucit performance parameters while keeping noise low.

For example you can increase the speed of a transimpedance amplifier without increasing its noise output with negative feedback.

to reduce the noise and for making system more stable use negative feedback..

to increase the stability of the negative feedback system.

here is a simple explanation. Suppose you have an op-amp (differental amplifier) with a gain of 40 db and a frequency response of -3db per octive falling off from 10 hz. That means that the gain will be:

HZ GAIN
10 40
100 37
1000 34
10,000 31
100,000 28

Voltage gain in DB is G=20 log (Vin/Vout) so voltage gain at 10 hz is 100 and voltage gain at 100,000 hz is only 25. (25.1 to be exact but whose counting?)

Now if you take that op amp and ground the + input, connect a resistor of 100 K from output to input, connect a resistor of 4 k from input to NEW CIRCUIT INPUT, you will reduce the gian of the resulting circuit to 25 (28 DB) BUT IT WILL BE 28 DB FROM 10 HZ TO 100,000 HZ. After that, the amp will be running "wide open" (no feedabck) so the response will fall off at 3 DB per octive. You have turned a useless op-amp (for audio purposes) into a fine wideband amplifier suitible for Hi-Fi reproduction.

All amplifiers are effectively single pole filters, that is, they exhibit a 3 DB per octive response at the best. So you can see, for effective wide band opereration, all useful amplifiers MUST have negative feedback.

Voltage gain in DB is G=20 log (Vin/Vout) so voltage gain at 10 hz is 100 and voltage gain at 100,000 hz is only 25. (25.1 to be exact but whose counting?)

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It should be 20log(Vout/Vin)

All amplifiers are effectively single pole filters, that is, they exhibit a 3 DB per octive response at the best. So you can see, for effective wide band opereration, all useful amplifiers MUST have negative feedback.

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In fact, high frequency communications such as optical transceivers used open loop amplifiers. Operating at such a high frequency (a few GHz), yet having negative feedback, the amplifier is just an oscillator!

It is really depends on what circuits you are constructing. For TIA, negative feedback helps to reduce current noise, hence improve sensitivity of the TIA. However, in other amplifiers, it could degrade input referred noise, as negative feedback in general reduces gain.